This invention relates to composite amplifiers including a plurality of parallelled amplifiers with Wilkinson-type combiners with isolation resistor stars, and particularly to such paralleled amplifiers in which the individual amplifiers or amplifier modules are coupled by switch networks to a combining node.
Many communication systems require transponders separated by significant distances. Such transponders when used for communication links between cities by radiation eliminate the need for land communication cables, which are very costly. The transponders cannot always be placed in the most ideal locations, but rather must be placed at locations where towers or other supports can be placed, and the antennas used with the transponders may be required to have high gain. High gain is achievable with antennas of reasonable size and cost only at microwave frequencies and at frequencies higher than microwave.
The transmission of signal from one transponder to another may require a power amplifier at the transmitting transponder which is capable of generating many watts of power with great reliability. In the past, microwave power was generated by traveling wave tubes (TWT). Traveling wave tubes were used, and continue to be used for microwave transponders notwithstanding the reliability problem attributable to the inherent degradation resulting from operation over a period of time.
More recently, solid state power amplifiers (SSPA) have been used instead of traveling wave tubes at lower microwave frequencies, such as C-band. The SSPA ideally has no inherent degradation mechanism, and is therefore more reliable then the TWT. This reliability is very advantageous, since microwave transponders are often placed at inaccessible locations, as for example at the tops of mountains. Generally speaking, solid state amplifiers are implemented by parallelling a relatively large number of low power solid state devices, amplifiers or amplifier modules. Each amplifier module contributes a portion of the total power output, and power combiners are used to combine the powers from each of the individual amplifier modules to generate the desired amount of sum signal power at the desired microwave or millimeter wave frequencies.
For purposes of reliability, it may be desirable to include within the SSPA one or more backup amplifier modules, which are switched into operation in the event of a failure of one of the other modules. Since some microwave transponders are located in areas far from a power grid, and therefore rely upon solar energy to generate energization voltages, it is important that such backup amplifier modules not be energized during those times when they do not contribute to the sum output power.
Various types of power combiners are described in the article "Microwave Power Techniques" by Kenneth J. Russell, published at pp. 472-478 in the IEEE Transactions On Microwave Theory and Techniques, May 1979. The Russell article describes corporate or tree combiners, in which chains of combinations are performed. Such arrangements tend to be disadvantageous because of the accumulation of losses in the combiners. It is very desirable that the power combination be performed with low loss. U.S. Pat. No. 4,641,106 issued Feb. 3, 1987, to Belohoubek et al. describes a low-loss radial combiner. The structure of radial power combiners is such that implementation of the switching circuits required for connection of redundant modules and disconnection of failed modules may be inconvenient.
U.S. Pat. No. 4,315,222, issued Feb. 9, 1982 to Saleh describes a power combiner arrangement in which the output power from a plurality of amplifier modules is combined at a single junction. Each amplifier module is coupled to the junction by a transmission line having an electrical length of one quarter wavelength (.lambda./4) at a frequency within the operating frequency range. The Saleh arrangement has the disadvantage that there is no isolation between the amplifier outputs, so that a change in output level or impedance at the output of a particular amplifier may affect the output power or tuning of the other amplifiers connected to the combining node.
FIG. 1 illustrates a combined amplifier including an input port 12 and an output port 14. A four-way power divider 16 is connected to input port 12 for receiving signal therefrom and for dividing the received signal into four portions of equal amplitude, which appear on conductors or transmission lines 18a, 18b, 18c and 18d. Each conductor 18a, 18b, 18c and 18d is connected to the input port of an amplifier module 20a, 20b, 20c and 20d, respectively. A combining node 22 is connected by an impedance transformer illustrated as a block 24 to an output port 14. Impedance transformer 24 may, as known, be a transmission line having a length equal to .lambda./4 (or odd integer multiples thereof) at a frequency near the center of the operating frequency band.
The output port of each of amplifiers 20a, 20b, 20c and 20d is connected by a length of transmission line 26a, 26b, 26c and 26d, respectively, to a set of single pole, single throw switches 28a, 28b, 28c and 28d, respectively. Switches 28a, 28b, 28c and 28d are connected in common to combining node 22. As illustrated in FIG. 1, switch 28a is nonconductive or open and switches 28b, 28c and 28d are closed or conductive.
With switches 28a, 28b, 28c and 28d in the positions illustrated, power can flow to combining node 22 from the output ports of amplifiers 20b, 20c and 20d. This situation might correspond to one in which amplifier 20a is held in reserve as a redundant amplifier, while amplifier modules 20b, 20c and 20d are on-line providing power to combining node 22 and, by way of impedance transformer 24, to output port 14. Those skilled in the art know that, by appropriate selection of the characteristic impedance of transmission lines 26a, 26b, 26c and 26d, together with selection of the length of each transmission line to be .lambda./4 at the center of the operating frequency band, as measured between the output port of each amplifier and combining node 22, a selected impedance may be presented to the output port of each amplifier and to output port 14. Instead of .lambda./4, odd integer multiples of .lambda./4 may be used. More specifically, proper selection of the lengths and characteristic impedances of the transmission lines can present a 50 ohm or 75 ohm impedance to both output port 14 and to the output ports of amplifiers 20a through 20d.
During those times when any one of amplifiers 20a through 20d is being adjusted or tuned, its output power may change, and/or its output impedance may change. As recommended in the above mentioned Russell article, a resistive star including resistors 30a, 30b, 30c and 30d may be connected between a floating node 32 and the output ports of each of amplifiers 20a through 20d. Those skilled in the art know that, so long as the signals produced at the outputs of amplifiers 20a through 20d are equal in amplitude and in-phase, node 32 will be at the same amplitude and a corresponding phase, so that no voltage appears across any of the resistors and no power is dissipated. However, changes in the output power or output impedance of any one amplifier as might be caused by tuning, degradation or other factors, results in current flow through one or more of the resistors which tends to absorb the incremental change, and thereby cancel the effect of a change at the output ports of the other amplifiers. This may be understood by considering that an increment of signal which might be produced at the output port of amplifier 20d travels to the output ports of amplifiers 20b and 20c by a first path including resistor 30d and resistors 30b and 30c, and also by a second path including transmission lines 26d, 26b and 26c. The path including the transmission lines has a total length of 2x (.lambda./4), or .lambda./2. The .lambda./2 path causes a phase inversion of the signal taking that path relative to the signal arriving by way of the resistors, which results in cancellation of the change as seen at the output ports of the associated amplifiers.
In the structure of FIG. 1, one of switches 28a, 28b, 28c and 28d will always be open, either because the amplifier associated with the open switch is a redundant amplifier awaiting insertion, or because it is associated with a failed amplifier which has been replaced by a redundant amplifier. As known to those skilled in the art, the impedance is zero at a point on a low-loss transmission line which is .lambda./4 from an open circuit. Thus, the impedance seen by the output port of amplifier 20a looking into transmission line 26a is a short circuit or at least a very low impedance. Consequently, the end of isolation resistor 30a which is connected to the output port of amplifier 20a is connected at a low impedance point. As a result, the desired signal voltage appears across isolation resistor 30a, and is thereby dissipated as heat rather than being coupled to output port 14. Another way of looking at the cause of the dissipation is that resistor 30a is connected in series with the parallel combination of resistors 30b, 30c and 30d, to form a voltage divider with node 32 at the tap, and the voltage divider is connected across the source of signal. However viewed, the attempt to use isolation resistors in conjunction with a switched arrangement as illustrated in FIG. 1 results in dissipation of the desired amplified signal in the isolation resistors.
FIG. 2 illustrates an arrangement for isolating the outputs of the amplifiers at the outputs of the amplifier modules from the effects of changes in the output signal from other amplifier modules. In FIG. 2, elements corresponding to those of FIG. 1 are designated by the same reference numerals. In FIG. 2, isolation between amplifiers is provided by a plurality of isolators 36a, 36b . . . 36d coupled between the outputs of amplifiers 20a, 20b . . . 20d, respectively and their output transmission lines 26a, 26b . . . 26d. As known, isolators 36a-36d circulate power between their input and output ports and one or more internal loads in such a fashion as to reduce or eliminate interaction between amplifiers. However, such isolators tend to be large, heavy and expensive. An improved arrangement for parallelling amplifier modules is desired in which small, light and low-cost isolation resistors may be used, without dissipating power in the isolation resistors when the amplifier with which they are associated is not contributing to the power combination.